Protective relay system



Jan) 5, 1943. F. GElSE PROTECTIVE RELAY SYSTEM Filed Oct. 30. 1940 .FZQ. 3

\* ll\v/ n l AvI\vA A A \AVAVI\\ I A A I I I} A A 1 v v v v WlTNESSESz' INVENTOR BY 2 Z 6 ATTORNEY Patented Jan. 5, 1943 t A v UNITED STATES PATENT OFFICE PROTECTIVE RELAY SYSTEM Friedrich Geise, Berlin-Karlshorst, Germany, as-

signor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 30, 1940, Serial No. 363,455 In Germany June 10, 1939 Claims. (Cl. 175294) The present invention relates to protective re- Fig. 2 is a wiring diagram showing another lay systems, and more particularly to a pilot wire embodiment of the invention. relay system which requires only two pilot wires. Figure 1 S O S a pilot Wire e y System in In pilot wire relaying systems for the protecaccordance with the present invention applied to tion of electrical transmission lines, the presence .3 e protection of a three-phase transmission line of a fault in a protected section of the line is desection I, which extends between suitable sectiontermined by comparing the current or voltage alizihg D tS indicated by the adjacent line conditions at opposite ends of the protected tions or bus bars 2 and 3. A circuit breaker 4 tion, and the system is thus enabled to distinguish having a trip c l 5 i p vided at each end of the between internal faults in the protected section line S n for diseenhectihg it from the adj and external or through faults. In order to cent sections.

make this comparison, the relaying equipment at The relaying equipment at both ends of the line Opposite ends of the protected line section is consection is identical and it will be described in denected together by pilot wires which t d from tall from one end only. Current transformers 6 one end to the other of the section, are provided at each end of the line section I In most types of pilot wire systems, three pilot in the s al manner to energize t y g sy Wires are required. Thus, in one well-known type tem, and they are connected in any suitable or of system, for example, the current transformers us l manner o a e-p se a a y ansat opposite ends of a protected line section are former l'. The current transformers 6 are conconnected together in series by means of two pilot nected to the primary winding 8 of the auxiliary wires, while dilferential relays at opposite ends of s or er d, as sho i t e d aWi y the section are connected in a bridge circuit by are Connected o unequally p d ps 0n the means of a third pilot wire. It is obviously depr m ry wi d o h a i l -ph l y n sirable to reduce the required number of pilot voltage app aerOss Secondary Winding 9 e wires since, in many cases, there are only a few a fault o y yp Occurs 0 e line e S communication channels available to serve as ondary windin s 9 0f t e a s m s l t 0D- pilot wires, and in any case the cost of the inposite ends of the line section i are connected tostallation would be materially decreased if only gether in series by means of pilot wires and two wires were needed. I I, which extend from one end to the other of the The principal object of the present invention 30 p te ed e Se Eao of the wind gs 9 is to provide a protective relaying yste of th has a plurality of taps, indicated at :2, and the pilot wire type in which only two pilot wires are pilot e l l is Connected to o Of e e t ps by a required, conductor I3, so that by changing taps the cur- A further object of the invention is to provide r nt in the pilo wires may be adjusted to any a pilot wire relaying system in Which a relay desired value. The other pilot wire I 0 is concircuit is employed at each end of a protected neoted to the secondary winding 9 in series with line section for tripping the line circuit breaker e pe a 1 1 Of an Ov e t el y I at that end, and is connected to the pilot wires and also in Series With a blocking e y IE. only when fault current fioWs in the line. The relay H5 has two oper g ls I7 and 1 A more specific object of the invention is to lo and it is designed to open its contacts :9 only provide a pilot wire relaying system in which only when current of a predetermined magnitude flows two pilot wires are utilized to connect the relayin the Same {inaction r g both coils mg equipment at opposite ends of a protected line and IS. A suitable electric dischar e device, such section, and having a tripping relay ci which as a glow lamp 20, 15 connected across the entne is connected to the pilot wires only when a fault M zii gg n g gg g g g g f g a: occurs, together with additional means for premaximum Voltage at maiy appear acrdss these venting iripplng of the l ne breakers on faults windings and also to p e the Circuit for the 22 32 0 5? ptrotecgedme i th coil M in certain cases, as will appear heyeina fte r 0 36 s an a Van ages 0 e mven A e y circuit is also rovided a each tion will be pparent from the following det il pp D t end of the protected line section and is arranged description, taken in connection with the accomto be connected to the system only when a fault D yin dra i in which: occurs on the line. This relay circuit includes a F gure 1 1s a wiring diagram showing one emtripping relay 2| having its coil 22 connected in bodiment of the invention; and series with a resistance 23. The relay circuit confore flow through these wires.

f coils I! and I8.

times the resistance of the pilot wires.

sisting of the relay 2| and resistance 23 is connected at one end to the conductor I3, and its other end is connected between the coils I1 and I8 of the blocking relay I6 whenever the relay I5 is energized and closes its contacts 24. The relay 2| has contacts 25 which are connected to complete a circuit through a conductor 26 for energizing the trip coil 5 of the circuit breaker 4 to cause tripping of the breaker. A glow lamp 21, or other suitable type of discharge device, may be connected across the pilot wires I0 and II at each end in order to protect the relaying equipment against high induced Voltages that may occur on the pilot wires.

The operation of this system is as follows:

Under normal conditions, all of the relays at each end of the line section I are deenergized andare in the position shown in the drawing, the

relays I5 and 2i having their contacts open while the relay It has its contacts closed. occurs on the line external to the protected line section I, fault current flows through the line section and is in the same direction at both ends. A single-phase relaying-quantity, in this case in the form of a voltage, appears across each of the secondaries of the transformers l, and since the fault current is flowing in the same direction at both ends of the line section, these voltages will be in the same direction. The secondaries 9 of the transformers I are connected in series through the pilot wires Ill and II, and

a relatively large circulating current will there- This current traverses the coil I4 of the overcurrent relay l5 and causes it to close its contacts 24, thus connecting the tripping relay 2| across the transformer I so that it is energized and closes its contacts 25. Simultaneously with the operation of the relay I5, however, the blocking relay I6 opens its contacts I9, since a relatively large current is flowing in the same direction through both the The contacts I9 are connected in series with the contacts 25 in the tripping circuit 26, and opening of these contacts, therefore, prevents tripping of the breaker, so that the line section I is not disconnected when an external 'fault occurs.

The resistance 25 is made relatively large compared to the resistance of the pilot wires II and I I for example, it may have aresistance of three For this reason, the current through the tripping relay circuit .is limited so that it is only large enough to operate the relay 2|, and the greater part of the currentcontinues to flow through the pilot wires IE] and II so that the blocking relay I6 is maintained energized to prevent tripping of the breaker. The discharge device 20 may break down and become conducting to limit the voltage across the winding 9 and the relay coil It to a safe value, but operation of the discharge device 20 does not affect the operation of the relay system since the voltage applied to the pilot wires is supplied from one of the taps, I2 through the conductor I3.

In case a fault occurs within the protected line section I, the voltages across the secondaries of the transformers 1 at opposite ends of the section oppose each other, and no current or only a very small current flows in the pilot wires II! and H.

' The voltage across the entire secondary winding 9 of each transformer, however, is high enough to cause'the discharge device 28 to become conducting, and a local circuit is thus completed through the discharge device 20 and the relay If a fault coil I4, causing the overcurrent relay I5 to close its contacts 24 and connect the tripping relay circuit across the transformer. As soon as the relay I5 closes its contacts, current flows through the coil I! of the relay IS, contacts 24 of the relay I5, operating coil 22 of the relay 2 I, the resistance 23, and conductor I3, back to the transformer 7. Thus, the relay 2| is energized and closes its contacts 25 to cause immediate tripping of the circuit breaker 4. Since there is little or no current flowing in the pilot wires, there will be substantially no current in the winding l8, and the relay It, therefore, is not energized, so that its contacts I9 remain closed, thus permitting im mediate tripping of the circuit breaker Q. If the fault current at one end of the line section I is much larger than at the other end, it may happen that the breaker at that end will be tripped first, but in any case both breakers will always be tripped in a positive and reliable manner' In case the fault current is supplied toflan internal fault from one end only of the line section I, the relay i5 at that end of the line section will be energized and close its contacts in the manner just described to energize the tripping relay 2i and cause tripping of the circuit breaker at that end. Even though there is no opposing voltage across the transformer I at the opposite end of the line section to prevent the flow of current through the pilot wires, there will be only a very small current because of the high impedance of the transformer winding at the opposite end, and thus the current in the relay coil I8 is not sufiicient to energize the blocking relay I6, and it remains deenergized with its contacts I9 closed to permit tripping of the breaker. Thus, only the breaker at the end of the line section from which the fault current is supplied is tripped, and even though there is no opposing voltage at the opposite end of the pilot wires-to prevent the flow of current in them, the blocking relay I6 is not actuated. This result is made possible by the fact that the tripping relay circuit at each end is energized only when fault current flows through the line section at that end. If the tripping relay circuit were permanently connected at each end, it is apparent that in case the fault current were supplied from one'end of the line section only, a current would flow through the pilot wires and through the tripping relay circuit at the other end, and thus the blocking relay at the first end of the line section would be energized and prevent tripping of the breaker. By connecting the tripping relay circuit only when a fault current flows atits end of the'line section, this difficulty is avoided and operation of the blocking relay is prevented.

Fig. 2 shows another embodiment of the invention which is particularly suitable when the protected line section is of great length, since it will operate satisfactorily with pilot wire currents as low as a few milliamperes. Fig. 2 shows the relaying equipment at one end only of a protected line section 30, only one phase being shown. As before, the line section has a circuit breaker 3| current relay 31, the contacts 38 of which are arranged to connect a tripping relay circuit across the transformer 34 when fault current flows in the line 30. The tripping relay circuit includes a tripping relay 39 having an operating coil 40 which is connected in series with a resistance 4| to a tap on the secondary winding 42 of the the relaying equipment from induced voltages that may appear on them.

Blocking means are also provided to prevent tripping of the breaker 3| on faults external to the protected line section 30. The blocking means consists of an overcurrent relay 41, which has its operating coil 48 connected in series with the pilot wire 43, and a wattmeter element 49. The contacts 50 of the wattmeter element 49 and the contacts 5| of the current relay 4'! are connected in parallel between the contacts 52 of the tripping relay 39 and the trip coil 32 of the line breaker 3|, so that when the contacts of both the wattmeter element 49 and the current relay 4'! are opened, tripping of the breaker is prevented. The voltage coil 53 of the Wattmeter element 49 is connected in series with the primary 35 of the transformer 34 and coil 36 of the relay'3l. The current coil 54 of the wattmeter element is connected in series with the pilot wire 43, and the wattmeter element is so adjusted that it opens its contacts when current flows in the same direction in both of the coils, and it is preferably adjusted to operate on a very small current in the coil 54.

It will be apparent that the operation of this system is essentially the same as that described above in connection with Fig. 1. Thus, when a fault occurs external to the protected line section 38, the fault current flowing through the line energizes the relay 31 and causes it to close its contacts 33, thus connecting the tripping relay circuit across the transformer 34 and causing the relay 39 to close its contacts 52. Since the voltages of the transformers 34 at opposite ends of the line section are in the same direction, however, current will flow through the pilot wires 43 and 4-4, thus causing both the relay 4! and the Wattmeter element 49 to open their contacts and prevent tripping of the breaker 3|.

In case of an internal fault within. the protected line section, the relay 31 is energized in the same manner as before, since its coil is connected to the primary of the transformer 34. Since the voltages of the transformers 34 at 0pposite ends of the line section now oppose each other, however, no current will flow in the pilot wires, and the relays 41 and 49 remain unenergized with their contacts closed, so that the breaker 3| is tripped as soon as the tripping relay 39 closes its contacts. In case the voltage across the transformer 34 at one end of the line section is greater than at the other, there may be a small current in the pilot wires which will be great enough to cause the wattmeter element 49 at one end of the line section to open its contacts. This current will not be large enough, however, to actuate the relay 4?, and since its contacts are in parallel with the contacts til of the relay 49, the tripping circuit is not interrupted and the breaker is immediately tripped. In case the fault is supplied from one end of the line section only, the impedance of the transformer at the opposite end will prevent the flow of any substantial amount of current in the pilot wires, and therefore the relays 41 and 49 will not be energized and tripping of the breaker will notbe prevented. It will be seen, therefore, that the operation of this embodiment of the invention is substantially the same as that of the previously described embodiment, but that because of the use of the wattmeter element 49 it can be made quite sensitive and thus satisfactory operation can be obtained with very small pilot wire currents, so that this embodiment of the invention is particularly suitable Where the protected line section 30 is very long.

It should now be apparent that a pilot wire relaying system has been provided which requires only tWo pilot wires, and which is very reliable in its action to insure immediate and positive tripping of the line breakers upon the occurrence of an internal fault in the protected line section, and which positively prevents tripping of the breakers in case of a fault external to the protected section.

It is to be understood that although two specific embodiments of the invention have been described for purposes of illustration, it is not limited to the exact arrangements shown, but

that various other modifications are possible, and in its broadest aspects, therefore, the invention includes all equivalent modifications and embodiments which come within the scope of the appended claims.

I claim as my invention:

1. A protective relaying system for a section of a polyphase electrical transmission line having circuit interrupting means at each end thereof, comprising transformer means including a secondary at each end of the protected line section for producing a single relaying voltage, pilot wires connecting said secondaries in series, a relay circuit at each end of the line section effective when actuated to cause tripping of the circuit interrupting means at that end, means at each end of the line section, energized by the associated secondary, for actuating said relay circuit when fault current flows in the line, and means for preventing tripping of the circuit interrupting means when the fault is external to the protected line section.

2. A protective relaying system for a section of a polyphase electrical transmission line having circuit interrupting means at each end thereof, comprising transformer means including a secondary at each end of the protected line section for producing a single relaying voltage, pilot wires connecting said secondaries in series, a relay circuit at each end of the line section effective when actuated to cause tripping of the circuit interrupting means at that end, an overcurrent relay connected in series with said secondaries and pilot wires, at each end of the line section for actuating the relay circuit at that end, a predetermined-response electric discharge device connected across a secondary and an overcurrent relay at one end of the protected line section, said electrical discharge devices being operable, in response to fault current in said line section, to operate said overcurrent relay, whereby said relay circuit is actuated, and a blocking relay at the end of the line section having the said electrical discharge device, connected in series with the pilot wires for preventing tripping of the circuit interrupting means when current flows in the pilot wires.

3. A protective relaying system for a section of a, polyphase electrical'transmissionline having circuit interrupting means at each end thereof, comprising transformer means including a primary and a secondary at each end of the protected line section for producing a single relaying voltage, pilot wires connecting said secondaries in series, a relay circuit at each end of the line section effective when actuated to cause tripping of thevcircuit interrupting means at that end, an overcurrent relay connected in series in the primary circuit of said transformer means for actu ating said rela circuit when fault current flows in the line, a wattmeter element having a first coilconnected in series with said overcurrent relay and a second coil in series with the pilot coil, and a relay, under control of said contact means, connected in series with the pilot wires, said wattmeter element and saidlast-mentioned relay being jointly effective to prevent tripping representative of line conditions at the associ-' ated end of the electrical apparatus, to produce a single electrical relaying-quantity in said secondary representative of line-conditions at that end; a pair of pilot wires serially connecting the said secondaries; a relay circuit at each end of the electrical apparatus, operative, when actuated, to cause tripping of the associated circuitinterrupting means; potentializing means at each end of said electrical apparatus, energizable by an excess value of said single electrical relayingquantity in the secondary thereat, for actuating said relay circuit; and blocking means energized by the single electrical relaying-quantities at both ,15 Wires, said wattmeter element having contact means operable when current flows in said second ends of theelectrical apparatusacting in combination through said pilot wires forpreventing under normal conditions and external fault conditions in said electrical apparatus actuation of either of said relay circuits by the associated potentializing means, but permitting actuation of the relay circuit at an end or ends of said electrical apparatus at which an excessive electrical relaying-quantity is produced by an internal fault in said electrical apparatus.

5. A protective relaying system for a line-section of a polyphase electrical apparatus having circuit-interrupting means in the power lines at each end thereof, comprising, at each end of said electrical apparatus, transformer means having a magnetic circuit and a secondary coupled thereto for magnetically combining electrical quantities representative of line conditions at the associated end of the electrical apparatus, to produce a single electrical relaying-quantity in said secondary representative of line-conditions at that end; a pair of pilot wires serially connecting the said secondaries; a relay circuit at each end of the electrical apparatus, operative, when actuated, to cause tripping of the associated circuitinterrupting means; potentializing means at each end of said electrical apparatus, energizable by an excess value of said single electrical relayingquantity in the secondary thereat for actuating said relay circuit; and blocking means energized by the single electrical relaying-quantities at both ends of the electrical apparatus acting in combination through said pilot wires for preventing under normal conditions and external fault conditions in said electricalapparatus actuation of either of said relay circuits by the associated potentializing means, but permitting actuation of the relay circuit at an end or ends of said electrical apparatus at which an excessive electrical relaying-quantity is produced by an internal fault in said electrical apparatus, said potentializing means being of a type which limits the excess value ofthe last said relaying-quantity.

FRIEDRICH GEISE. 

